Microwaves are a form of radio waves generally considered to include wavelengths ranging from approximately 1 millimeter (mm) to 1 meter (m) in length. Microwaves are generally considered to include electromagnetic energy ranging in frequency between 300 megahertz (MHz) and 300 gigahertz (GHz). Commonly, microwave applications utilize electromagnetic energy ranging from approximately 1 GHz to approximately 94 or 110 GHz. More commonly, microwave applications utilize electromagnetic energy ranging from approximately 1 GHz to approximately 67 GHz; however, in microwave applications can also use other ranges of electromagnetic energy.
Microwaves are used in the fields of communication, satellites, radar, radio astronomy, navigation, heating and power applications, and spectroscopy. More specifically, microwaves are used extensively in telecommunications for non-broadcast, point-to-point uses. Microwaves are especially suitable for point-to-point uses because microwaves are more easily focused into narrow beams and require smaller antenna sizes than lower frequency radio waves and provide for broad bandwidth and high data flow. Additionally, microwaves are commonly used for transmitting data for television and telephones both between ground stations and between satellites.
Microwaves are also transmitted or propagate through transmission lines. Exemplary transmission lines include single conductor transmission lines such as rectangular waveguides or multiple conductor transmission lines such as microstrips, strip lines on printed circuit boards, and coaxial (or “coax”) cables and connectors. Coaxial connectors include an inner conductor surrounded by a tubular insulating or dielectric layer. The inner conductor and insulating layer are surrounded by a tubular outer conductor or shielding layer such that the inner conductor and outer conductor share a geometric axis along a length of the transmission line. Coaxial connectors can optionally include an outer insulating jacket or sheath.
Coaxial connectors generally include circular cross-sectional areas (taken transverse to the axis of the inner and outer conductors), but can also include cross-sectional areas of any shape. Tolerance for dimensions of coaxial connectors are precisely controlled to maintain constant spacing between the inner and outer conductors. Constant spacing between the inner and outer conductors is important because in an ideal coaxial connector a signal carrying electromagnetic field exists only in the space between the inner conductor and the outer conductor. Carrying the signal in the space between the inner and outer conductors of the coaxial connector allows the signal to be shielded or protected from external electromagnetic interference by the outer conductor.
Additionally, an electric field interaction in the coaxial connector caused by propagation of the signal creates a distributed capacitance (C) between the inner and outer conductors. The capacitance is influenced by a number of factors, including a proximity of the inner and outer conductors, relative surface areas of the inner and outer conductors, and a dielectric constant of the material disposed between the inner and outer conductors. Similarly, a magnetic field interaction in the coaxial connector caused by propagation of the signal creates a distributed inductance (L) between the inner and outer conductors. The inductance is influenced by a number of factors, including the proximity of the inner and outer conductors, the relative surface areas of the inner and outer conductors, and the dielectric constant of the material disposed between the inner and outer conductors. An impedance (Z) of the signal line is a function of both the capacitance and inductance. Stated more precisely, the impedance of the line signal is equal to a square root of the inductance divided by the capacitance: Z=√{square root over (L)}/C. The relationship between the impedance, inductance, and capacitance of a microwave line signal creates a number of challenges for the design of microwave connectors.
One challenge presented by the use of coaxial connectors in systems including microwave line signals is controlling and minimizing reflections of the microwave signal caused by the connectors. Reflections caused within microwave signal connectors can result in part from transmission lines of different diameters being connected to the microwave signal connectors. Systems including more than one size of transmission cables or devices to be connected are not uncommon. Because microwave signal connectors include ends that physically and electrically match the cables or devices being connected, microwave signal connectors also include different diameters, as needed, to satisfy system needs. When a first end of a microwave signal connector includes a first size different from a second size of a second end of a microwave signal connector, a transition between the first and second sizes occurs within the microwave signal connector.
In accounting for the transition between the size of the first and second ends, microwave signal connectors are constructed such that an impedance of the microwave signal connector is constant at the first and second ends of the microwave signal connector. However, because impedance of the signal line is related to a capacitance and inductance of the signal line by the relationship of Z=√{square root over (L)}/C, the capacitance and inductance associated with each of the first and second sizes of the first and second ends of the microwave signal connector change and are not equal even when impedance through the microwave signal connector is kept the same. The variation or change of the inductance and especially the variation or change of capacitance in a microwave signal connector cause portions of the microwave signal propagating through the microwave signal to be reflected. Reflections of portions of the microwave signal are undesirable because the reflections degrade or weaken a strength of the signal being transmitted. Therefore, compensation steps are used to minimize reflections of the microwave signals being transmitted through microwave signal connectors.
FIG. 1 is a diagram of an exemplary microwave signal connector 10 as known in the prior art. FIG. 1 illustrates details relating to a number of performance related details without disclosing mechanical engagement of the various features. Microwave signal connector 10 includes a connector portion 10a and a transition portion 10b. Microwave signal connector 10 includes a first end 12 configured to be attached to a transmission line such as a coaxial cable, and a second end 14 configured to be attached to a transmission line such as microstrip 16. Microstrip 16 includes a conductive material or waveguide 18 formed at or on a substrate 20. Microwave signal connector 10 is optionally secured to microstrip 16 with securing portion 22. While microwave signal connector 10 is shown with first end 12 configured to be attached to a coaxial cable and second end 14 configured to be attached to microstrip 16, the microwave signal connector can likewise be configured with first and second ends that are configured to be attached to other transmission lines or coaxial cables of different sizes.
Microwave signal connector 10 includes an inner conductor 26 that extends from first end 12 to second end 14 of the microwave signal connector 10. Inner conductor 26 is bisected by central axis 28 of microwave signal connector 10 and includes a first diameter d1 that extends from first end 12 partially but not completely through the microwave signal connector. The diameter of inner conductor 26 decreases in size in a stair-step fashion to diameters of decreasing size. Specifically, FIG. 1 shows the diameter of inner conductor 26 decreasing from d1 at first end 12 to d2, d3, d4, and to d5 at second end 14, where inner conductor 26 contacts transmission line 18.
Microwave signal connector 10 further includes an outer conductor 30 that extends from first end 12 to second end 14 of the microwave signal connector. Outer conductor 30 is bisected by central axis 28 of microwave signal connector 10 such that outer conductor 30 is positioned with the same central axis as inner conductor 26, that is coaxially. Outer conductor 30 includes a first inner diameter D1 that extends from first end 12 partially but not completely through microwave signal connector 10. The inner diameter of outer conductor 30 decreases in size in a stair-step fashion to diameters of decreasing size. Specifically, FIG. 1 shows the diameter of outer conductor 30 decreasing from D1 at first end 12 to D2, D3, D4, and to D5 at second end 14. Optionally, outer conductor 30 includes securing portion 22.
The stair-step decrease in diameters of inner conductor 26 and outer conductor 30 are offset with respect to one another such that an offset K1 exists between the change in diameter from d1 to d2 of inner conductor 26 and the change in diameter from D1 to D2 in outer conductor 30. Similarly, offsets K2 and K3 are shown at the transitions from d2 to d3 and D2 to D3 as well as from d3 to d4 and D3 to D4, respectively.
Dielectric materials 34, 36, and 38 are disposed between inner conductor 26 and outer conductor 30. Dielectric materials 34, 36, and 38 are homogenous dielectric materials and include both air and plastics that maintain consistent dielectric properties and provide low attenuation of electromagnetic energy over large ranges of operating frequencies. In one common embodiment, dielectric materials 34 and 38 are air, and dielectric material 36 is a plastic material. Plastic material 36 includes thermoplastics such as polyethermide (also known as Ultem®), polyether ether ketone (PEEK), polychlorotrifluoroethylene (PCTFE) (also known as Kel-F®), and fluoropolymers such as polytetrafluoroethylene (also known as Teflon®). Dielectric materials 34, 36, and 38 are disposed between offsets K1, K2, and K3. Adjustments to a length or distance of offsets K1, K2, and K3 are made in order to adjust capacitance and inductance within the microwave signal connector 10 and to minimize reflections of the microwave signal being transmitted through the microwave signal connector. A dielectric constant of dielectric materials 34, 36, and 38 is adjusted or changed in order to adjust capacitance and inductance within the microwave signal connector 10 and to minimize reflections of the microwave signal being transmitted through the microwave signal connector. The governing equations used to minimize microwave signal reflections by adjusting the dielectric constants of dielectrics 34, 36, and 38 and by adjusting the offsets K1, K2, and K3 are well known in the art and are based on the use of homogenous or unitary dielectric materials disposed between inner conductor 26 and outer conductor 30.
Details relating to known means and methods of mechanical engagement for microwave signal connectors, omitted from the functional description made above in reference to the various features of FIG. 1, are now addressed. A primary objective for microwave signal connectors is to maintain the position and structural integrity of the inner conductor and dielectric material within the outer conductor when assembled to transmission lines and other devices. Current construction methods, as known in the prior art, use purely mechanical means for holding the microwave signal connector together. Known construction methods for mechanically holding the microwave signal connector together include construction using barbs and dimples, beads, and an epoxy rod for mechanical capture.
A known construction method for mechanically holding a microwave signal connector together includes the use of barbs and dimples. Barbs are used to capture an inner conductor within a plastic dielectric material by deforming or displacing the plastic to receive the inner conductor and then apply pressure to the inner conductor. Dimples are formed in the outer conductor that deform a portion of the outer conductor to capture the plastic dielectric material by extending the outer conductor into, and applying pressure on, the plastic dielectric material. While the barbs and dimples mechanically secure the inner conductor, the plastic dielectric, and the outer conductor to one another, the changes in geometry of the transmission line resulting from the deformation caused by the barbs and dimples undesirably cause reflections of microwave energy during transmission of a microwave signal.
Another known construction method for mechanically holding a microwave signal connector together includes the use of beads. A bead of a harder plastic material including thermoplastics such as polyethermide, PEEK, and PCTFE are used to capture the inner and outer conductors. Grooves are formed or machined into the outer and inner conductors to receive a portion of the bead. The bead can be pressed into the grooves, or alternatively, a subsection of one or more conductors can be screwed together with the bead. In either event, the conductors and bead are captured between the inner conductor and the outer conductor, thereby locking and holding the microwave signal connector together.
Another known construction method for mechanically holding a microwave signal connector together includes the use of an epoxy rod. The epoxy rod is made by forming or drilling holes completely through an outer conductor and completely through a plastic dielectric. A hole or groove and is formed or machined partially through an inner conductor to form a groove. The holes or openings in the outer conductor, dielectric, and inner conductor are aligned and a liquid epoxy is injected into the aligned openings and cured to form a solid rod. The epoxy rod mechanically captures or fixes the dielectric in place relative to the inner and outer conductors by being rigidly fit within the holes made through the outer conductor, dielectric, and inner conductor. Thus, the rod-like shape of the cured epoxy is used to mechanically secure together the components of the microwave signal connector.